1. Field of the Invention
The present invention relates to vacuum pumps used for nuclear power industry and, more specifically, to oilless double-wrap dry scroll vacuum pumps, comprising a pair of stationary scrolls and a revolving scroll, the revolving scroll being driven without contact to an external driving source.
2. Description of the Related Art
A scroll vacuum pump comprises a stationary scroll having a base and a scroll wrap formed thereon, a revolving scroll having basically the same shape as the stationary scroll, engaging said stationary scroll out of phase 180 degrees and being revolved by a crankshaft, said crankshaft and an rotation preventing mechanism. The pump operates to make vacuum the suction side of it by means of the change in volume of a crescent sealed space (i.e. a compression chamber) formed between the helical wrap of the stationary scroll and that of revolving scroll as the revolving scroll moves relative to the stationary scroll. FIGS. 7(a), 7(b), 8(b) and 8(c) illustrate the operation of the pump mechanism. In a state shown in FIG. 7(a), a space between the outer side of the revolving scroll wrap 150a and the stationary scroll wrap 151 is closed to end a suction step, thus the gas introduced through a suction port 152 in a compression chamber 153 as shown as a dotted area.
In a subsequent state shown in FIG. 7(b) after the phase advancement of a crankshaft (not shown) by 90 degrees, a suction step in a space 154 formed between the outer side 150a of the revolving scroll wrap and the inner side of the beginning portion of the stationary scroll wrap 151 sets in, a compression step sets in in the intermediate compression chamber 155, and a step of discharging through a discharging port 157 sets in in a compression chamber located at the center of the base.
FIGS. 8(b) and 8(a) show subsequent states after every 90 degree phase advancement of the crankshaft which is rotating clockwise.
With the revolving of the revolving scroll, the compression chamber 153 shown as the dotted area shift toward the center of the scroll and gradually reduced in volume to compress gas. Through the states shown in FIG. 8(a) and 7(a), the gas is discharged through the discharge port 157 which is provided in a central portion of the stationary scroll.
As shown above, the suction gas is continuously compressed, and neither suction valve nor discharge valve is necessary. As described before in FIGS. 7 and 8, the scroll vacuum pump has the following merits.
a. Since a plurality of compression chambers are formed and the suction, compression and discharging steps are executed simultaneously and continuously, the torque fluctuation is little. Hence vibration and noise are low. PA1 b. Since a plurality of compression chambers are formed between the suction port and the discharging port, the pressure difference among adjacent compression chambers is low. Hence gas being compressed does not leak greatly. PA1 c. As the radius of motion of movable part is small and the speed of frictional motion is low the wear resistance is high. PA1 1) a gas-tight structure which isolates a pump body from the outside and a structure for preventing leakage of gas from a compression passage to the outside of the pump in order to eliminate radioactive pollution during operation, PA1 2) an oilless bearing for securing improved durability thereof, attaining long-term non-stop operation and preventing deterioration of the heat transfer performance due to intrusion of oil in low pressure parts of the pump and PA1 3) an efficient cooling mean. PA1 a) To attain the structure described in 1) above, indirect torque transmitting means such as a magnetic coupling for separating the pump body from driving mechanism are provided. PA1 b) To attain the functions described in (2) and (3), a gas bearing is adopted, and gas passing through a passage in the gas bearing is effectively utilized for the cooling of a revolving scroll drive shaft. PA1 the pump body further comprises:
Furthermore, the number of components of the pump is small.
The scroll vacuum pump mentioned above is of a single wrap dry type. Recently, a double wrap dry type vacuum pump, which comprises a revolving scroll having a base supported on a crank shaft and a pair of scroll wraps provided on the both sides of the base in the axial direction thereof, and a pair of stationary scrolls each having a scroll wrap engaged with each of the both scroll wraps of the said revolving scroll, tends to be used owing to their superior efficiency.
Generally in a scroll fluid machine including a scroll compressor, fluid sucked from the outer periphery is compressed in sealed spaces formed between the stationary and revolving scrolls as it is successively carried toward the machine center, and the compressed fluid is discharged from the center part.
This machine, compared to other types of compressors, exerts high efficiency as it has such merits that the compression process is continuous, neither suction valve nor discharging valve is necessary, the torque fluctuation is little, leakage from compression chambers is not great. Furthermore the speed of frictional motion of frictional part is low, and the number of components is small. Fields of its application to utilize its high efficiency, low vibration level, low noise level and high reliability are being developed, and it is utilized not only in coolant compressors but also in air compressors, helium compressors and vacuum pumps for nuclear power purposes.
Meanwhile equipment of nuclear power industry is required to perfectly prevent it's influence on other related equipment and to be highly durable and reliable.
The nuclear power equipment, unlike general equipment, is necessary to exert high performance and high reliability. Particularly, environmental pollution by radioactive substances owning to related nuclear power equipment during operation should perfectly be prevented. In addition, it is required to form a boundary zone which is isolated from external environments and in which external environments can not affect other equipment connected to the said equipment.
For the above reasons, vacuum pumps used for vacuum vessels in nuclear power industry are requisite to prevent radioactive pollution during operation and have radioactive resistance and wear resistance so as not to deteriorate constituents of the equipment. It is thus necessary to select isolating means and cooling means by taking the above requirements into considerations. Particularly, it is required to ensure high degree of vacuum, ensure getting rid of various troubles due to oil and provide satisfactory seal structure, bearing structure for long-term non-stop operation.